The invention relates to a process for the recovery of ammonia from waste water containing NH3, at least one acid gas (CO2 and/or H2S) and inert gases, which is firstly passed through a pretreatment column and then applied, at least in part, to a total stripping column, in which process a top product from the total stripping column which comprises NH3 and acid gas is fed to a condenser, in which the top product is scrubbed with circulated and cooled condensate, an aqueous NH3-containing condensate coming from the condenser is fed to an NH3 stripping column, whose top product is brought into direct contact with circulating aqueous NH3-containing condensate in a wash column, NH3 is obtained from the top product from the wash column, and some of the bottom product from the wash column is fed back into the NH3 stripping column. The waste water, which always comprises at least one of the acid gases CO2 and/or H2S, may also comprise, for example, HCN.
A process of this type is disclosed in EP-B-0 212 690. The pretreatment column here is designed as a simple stripping column, and consequently the treated waste water still contains a considerable part of the impurities that are difficult to remove. This results in ammonia water finally obtained likewise having a relatively high content of impurities.
The invention has the object of modifying the known process in such a way that the burden on the wash column upstream of the NH3 recovery is reduced, and clean, aqueous NH3 can be produced. This is achieved according to the invention in the process mentioned at the outset in that the pretreatment column is provided with heating of the bottom region, and the temperature in the bottom region is from 130 to 200xc2x0 C., in that a sub-stream of the waste water is introduced into the upper region of the pretreatment column and a second sub-stream of the waste water is passed into the pre-treatment column below the first sub-stream, in that at least 80% and preferably at least 95% of the NH3 present in the top product from the total stripping column is condensed in the condenser, and in that a waste-water stream is taken off from the total stripping column, at least part of the waste-water stream is cooled to temperatures of from 10 to 60xc2x0 C., and the cooled waste-water stream is passed into the top region of the pretreatment column.
Inert gases and acid gases (CO2 and H2S) are removed very effectively in the pretreatment column, which reduces the burden on the downstream columns. This results, inter alia, in it being possible to condense all of the top product from the total stripping column. It is also possible to adjust the pressure in the wash column while nevertheless obtaining clean ammonia water.
The pressure in the pretreatment column is usually in the range from 1 to 20 bar and is preferably at least 2 bar. The second sub-stream of the waste water, which is passed into the pretreatment column below the feed point of the first sub-stream, is preferably preheated to temperatures of at least 50xc2x0 C. and preferably at least 80xc2x0 C. by indirect heat exchange with the waste water coming from the bottom of the pretreatment column.
It is advantageous to pass from 1 to 40% of the waste water taken off from the bottom of the pretreatment column into the condenser while bypassing the total stripping column. In this way, the load in the total stripping column can be regulated and its heating demand optimized.
Possible embodiments of the process are explained with the aid of the drawing, which shows a flow chart of the process.
The waste water to be treated is supplied in line (1). It comes, for example, from a refinery or a plant for coal gasification. The waste water comprises NH3 as valuable substance and in addition numerous further components, in particular one or more acid gases, such as CO2 and/or H2S, also inert gases and possibly HCN and also residues of hydrocarbons or solvents. A first sub-stream of the waste water is introduced through line (1a) into the upper region of a pretreatment column (2). The remaining waste water is fed through line (1b) and, before entry into column (2) through line (1c), for example in its central region, is warmed to at least 50xc2x0 C. and preferably at least 80xc2x0 C. in the indirect heat exchanger (3). The pretreatment column (2) and the other columns contain trays known per se or alternatively packing elements. The bottom region of column (2) is provided with heating (4), enabling temperatures of from 130 to 200xc2x0 C. to be achieved therein.
Some of the waste water obtained in a total stripping column (10) is fed through lines (5) and (5b) to the top of column (2) after the waste water has been passed through a condenser (6) and adjusted to temperatures in the range from 10 to 60xc2x0 C. A further waste-water stream comes from an NH3 stripping column (30) and is fed through line (8) to the lower part of column (2). Stripped gases and vapors leave column (2) in line (7).
If necessary, a water-containing liquid stream comprising hydrocarbons and/or solvents is taken off through line (35) and fed to work-up, which is not shown.
The waste water obtained at the bottom of column (2) is taken off in line (9), fed through heat exchanger (3) for cooling and then applied to the total stripping column (10) in lines (11) and (11a). In column (10), it is ensured that all the free NH3 and remaining acid gases are removed from the waste water. To this end, column (10) is fitted with bottom heating (12), by means of which the bottom liquid is brought to temperatures of from 100 to 180xc2x0 C. Column (10) furthermore contains top cooling (13). It may be advantageous from a control engineering point of view to branch off a sub-stream of from 1 to 40% of the waste water coming from the heat exchanger (3) and supplied in line (11) and to feed it through line (15) into the condenser (20). A sub-stream of the waste water taken off from column (10) is removed through line (5a) and can be fed, for example, to biological waste-water treatment.
The NH3-rich top product from column (10) is fed through line (14) into a condenser (20), which is likewise fitted with trays or packing elements. Condensate coming from condenser (18) and partially circulated through lines (17) and (17b) is introduced into the upper region of condenser (20) through line (17c). The gas mixture taken off from condenser (20) in line (19) comprises mainly inert gases.
The condensate in line (17) is split over lines (17a) and (17b). Line (17a) leads to the NH3 stripping column (30), which is likewise fitted with heating (31). The bottom liquid from column (30) is fed back to the pretreatment column (2) through line (8) in the manner already explained. The top product is introduced through line (22) into the wash column (25), whose design and mode of operation is described in detail in EP-B-0 212 690. Some of the liquid flowing out in column (25) is fed back into column (30) through line (23), and the remaining liquid is fed back into the lower region of column (25) by means of the circuit through line (24) and condenser (26). Column (25) is preferably designed as a Wetted-wall column divided into several sections, the pressure being in the range from 1 to 20 bar and the temperatures being from 20 to 100xc2x0 C.
A gas mixture consisting principally of NH3 is taken off from the top of column (25) in line (28) and fed to ammonia liquefaction (29), from which liquid ammonia is taken off in line (34) and/or ammonia water is taken off in line (32). Some of the ammonia water is fed back to the top of column (25) through line (33), and the remainder is available in line (32a) as a further valuable product. It is possible to generate either aqueous or liquid ammonia or both products. If necessary, water is supplied in line (36).